Method and system for base station change of packet switched communications in a mobile communications system

ABSTRACT

The present invention relates to change of base stations transferring packet switched communications between a mobile station and a support node. The base station change is of lossless type allowing lossless base station change of packet switched communications in unacknowledged mode between the mobile station and the support node.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to communications. More especially itrelates to packet data communications over radio links and change ofbase station. Particularly it relates to base station handover of packetswitched communications in GPRS (General Packet Radio System) and UMTS(Universal Mobile Telecommunications System) communications.

BACKGROUND AND DESCRIPTION OF RELATED ART

Packet Radio Services offers packet switched communications over radiolinks in e.g. GPRS and UMTS. Data is disassembled and transmitted inpackets or Protocol Data Units (PDUs). Upon reception, the PDUs arereassembled.

FIG. 1 illustrates protocol layers for GERAN (GSM-EDGE Radio AccessNetwork) A/Gb mode and will be explained in some detail below. Allfunctions related to transfer of Network layer Protocol Data Units,N-PDUs, shall be carried out in a transparent way by the GPRS networkentities.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup GERAN, Digital cellular telecommunications system (Phase 2+);General Packet Radio Service (GPRS); Overall description of the GPRSradio interface; Stage 2 (Release 4), 3GPP TS 43.064 V4.3.0, France,February 2002, provides the overall description for lower-layerfunctions of GPRS and EGPRS (Enhanced GPRS) radio interface, Um. In thesequel GPRS refers to both GPRS and EGPRS in not explicitly statedotherwise. An EGPRS mobile/base station is a GPRS compliant mobile/basestation with additional capabilities for enhanced radio access protocolfeatures and enhanced modulation and coding schemes. The support ofEGPRS is optional for mobile station and network.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup GSM/EDGE Radio Access Network; General Packet Radio Service(GPRS); Mobile Station (MS)—Base Station System (BSS) interface; RadioLink Control/Medium Access Control (RLC/MAC) protocol (Release 4), 3GPPTS 44.060 V4.8.0, France, September 2002, specifies the procedures usedat the radio interface for the General Packet Radio Service, GPRS,Medium Access Control/Radio Link Control, MAC/RLC, layer. The RLC/MACfunction supports two modes of operation:

-   -   unacknowledged operation; and    -   acknowledged operation.

Section 9.3 describes operation during RLC data block transfer. RLCacknowledged mode, RLC-AM, operation uses retransmission of RLC datablocks to achieve high reliability. RLC unacknowledged mode, RLC-UM,operation does not utilize retransmission of RLC data blocks.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Radio Access Network, Physical Layer Procedures, 3G TS 25.322v3.5.0, France, December 2000, specifies three data transfer services ofradio link control, RLC:

-   -   transparent data transfer service,    -   unacknowledged data transfer service, and    -   acknowledged data transfer Service

Subsections 4.2.1.1 and 4.2.1.2 describe transparent mode entities andunacknowledged mode entities. One difference of the two modes resides inmanagement of packet overhead. In transparent mode no overhead is addedor removed by RLC. In subsection 4.2.1.3 an acknowledged mode entity,AM-entity, is described (see FIG. 4.4 of the 3GPP TechnicalSpecification). In acknowledged mode automatic repeat request, ARQ, isused. The RLC sub-layer provides ARQ functionality closely coupled withthe radio transmission technique used.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Core Network; Digital cellular telecommunications system (Phase2+); Mobile Station (MS)—Serving GPRS Support Node (SGSN); SubnetworkDependent Convergence Protocol (SNDCP) (Release 5), 3G TS 44.065 v5.1.0,France, September 2003, provides a description of the SubnetworkDependent Convergence Protocol, SNDCP, for GPRS. SNDCP entity performsmultiplexing of data coming from different sources to be sent usingservice provided by the LLC (Logical Link Control) layer,

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Core Network; General Packet Radio Service (GPRS); GPRS TunnellingProtocol (GTP) across the Gn and Gp interface (Release 5), 3GPP TS29.060 V5.8.0, France, December 2003, defines the second version of GTPused on:

-   -   the Gn and Gp interfaces of the GPRS;    -   the Iu, Gn and Gp interfaces of the UMTS.

Within GPRS (and UMTS) Gn interface is an interface between

GPRS Support Nodes (GSNs) within a PLMN and Gp interface is an interfacebetween GPRS Support Nodes (GSNs) of different PLMNs. In UMTS Iuinterface is an interface between RNC and Core Network.

A Gb interface is an interface between an SGSN (Serving GPRS SupportNode) and a BSC (Base Station Controller). An A interface is aninterface between BSC and MSC (Mobile Services Switching Center).

GPRS Tunneling Protocol, GTP, is the protocol between GPRS SupportNodes, GSNs, in the UMTS/GPRS backbone network. GTP allowsmulti-protocol packets to be tunneled through the UMTS/GPRS Backbonebetween GSNs and between SGSN (Serving GSN) and UTRAN (UniversalTerrestrial Radio Access Network).

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Core Network; Mobile Station—Serving GPRS Support Node (MS-SGSN);Logical Link Control (LLC) layer specification; (Release 4), 3GPP TS44.064 V4.3.0, France, March 2002, defines the Logical Link Control,LLC, layer protocol to be used for packet data transfer between theMobile Station, MS, and Serving GPRS Support Node, SGSN. LLC spans fromthe MS to the SGSN. LLC is intended for use with both acknowledged andunacknowledged data transfer.

LLC supports two modes of operation:

-   -   Unacknowledged peer-to-peer operation, LLC-UM, and    -   Acknowledged peer-to-peer operation, LLC-AM.

In unacknowledged operation logical link entity may initiatetransmissions to a peer entity without prior establishment of a logicalconnection with the peer entity. LLC does not guarantee in-orderdelivery. LLC can detect errors in a received frame, and, depending onwhether the frame is sent in protected mode or not, either discard ordeliver the erroneous frame. No error recovery procedures are defined atthe LLC layer. Higher-layer protocols can be used to providereliability, if needed. This mode of operation is known as AsynchronousDisconnected Mode, ADM.

With acknowledged operation a balanced data link involves twoparticipating entities, and each entity assumes responsibility for theorganization of its data flow and for error recovery proceduresassociated with the transmissions that it originates. Each entityoperates as both a data source and data sink in a balanced link,allowing information to flow in both directions. This mode of operationis known as Asynchronous Balanced Mode, ABM, and provides a reliableservice with in-order delivery.

European Patent Application EP1318691 describes a method for informingthe SGSN about a mobile station cell-change operation in the GPRS.

International Patent Application WO03032672 discloses a method ofoptimization of handover procedures in GPRS comprising the old SGSNsending identification response directly to the new SGSN.

International Patent Application WO0079808 claims a method of reducingdelay time for a mobile station being handed over from an old SGSN to anew SGSN during a call handling a real-time payload in a GPRS packetswitched radio telecommunications network comprising shortening theinter-SGSN Routing Area Update interruption interval and implementinglow latency requirements and shaping of packet traffic.

International Patent Application WO02085048 describes a handoverprocedure for use in a GPRS network, reducing the need for re-sequencingin SGSN. Old SGSN sends a message to GGSN (Gateway GSN) requesting datatransmission to stop. Data at old SGSN, for transmission to MS, istransferred to new SGSN and transmission from GGSN is resumed whenhandover is complete. GGSN then transmits data to new SGSN.

In U.S. Patent Application US20010019544 the GGSN and SGSN are allowedto finish up on-going transactions before moving the context to the newSGSN. The first (old) SGSN is operating as a temporary anchor inresponse to an inter-SGSN routing area update.

European Patent Application EP1345463 reveals buffering of TCP packetsin a mobile node during handover.

FIG. 2 illustrates schematically some network elements involved inpacket switched handover. A source SGSN <<source SGSN>> connected to agateway GSN <<GGSN>> supports data traffic to a mobile station <<MS>>via a source base station subsystem <<source BSS>>. A base stationchange may be initiated, e.g. as the mobile station moves, towards abase station of a target base station subsystem <<target BSS>> supportedby a target SGSN <<target SGSN>>.

In prior art lossy type of packet switched handover is used for servicesrequiring short delay but allowing some data loss at cell change, e.g.speech services. For lossy handover downlink data is typicallyduplicated by the source SGSN and sent both to the source BSS forfurther transmission to the mobile station in the current cell, and tothe target SGSN <<target SGSN>>.

The target side (BSS/SGSN) can either discard the forwarded data untilthe MS has indicated its presence in the target cell or, blindly, sendthe data without information available on whether or not the MS ispresent in the target cell. In case of blindly sending the data, themobile station has been ordered to perform the handover and hassynchronized towards the target cell, the downlink data flow is alreadyongoing and the mobile station can immediately start the uplink dataflow. No acknowledgement of received data is required, neither in uplinknor in downlink.

According to prior art solutions, data losses will occur, e.g., whendata packets sent to source BSS from source SGSN are discarded in sourceBSS when a mobile station is handed over from source BSS to target BSS.Losses will also occur if e.g. packet data forwarded to MS via thetarget SGSN and target BSS experiences a delay that is less than thedelay associated with the MS processing the handover command andacquiring synchronization.

Lossless type of packet switched handover, PS handover, is used forservices that are sensitive to data losses but can accept a certaindelay. The typical characteristics of a lossless handover are currentlybased on acknowledged RLC and LLC protocols and the SNDCP protocoloperating in acknowledged mode. During the PS handover the downlink dataflow is forwarded from the source SGSN to the target SGSN. The targetSGSN buffers the downlink data until the mobile station has indicatedits presence in the target cell. The SNDCP layer at both the MS and SGSNassigns N-PDU Send number to each N-PDU sent and maintains N-PDU Receivenumber for each received N-PDU for any given bi-directional packetservice. When a handover is performed of such a service the number ofthe next expected uplink and downlink N-PDU is exchanged between the MSand the SGSN in handover signaling messages allowing precise knowledgeof where packet data transmission should resume after handover.

None of the cited documents above discloses lossless packet switchedbase station handover or radio cell change in LLC unacknowledged mode,LLC-UM.

SUMMARY OF THE INVENTION

Packet switched base station handover according to the invention isassociated with cell change both within GERAN and between GERAN andUTRAN. According to the invention the cell reselection time can bereduced. Also, due to the invention allowing operating, particularlyLLC, in unacknowledged mode, link delay can be reduced during an entiredata transfer session. This is particularly the case for base stationhandover in RLC-UM and LLC-UM.

Handover for which data losses may occur is known as lossy handover. Forpacket data communications with strict delay requirements losslesshandover according to prior art is not always feasible due to itsimposed additional delay caused by acknowledgements and retransmissions.The delay introduced by particularly LLC protocol layer in acknowledgedmode affects higher layer operations, e.g. TCP based services with aresulting throughput deterioration due to TCP congestion controlerroneously interpreting the additional delay as channel congestion.Presently the only packet switched handover solution offered for delaysensitive applications, when LLC-AM is excluded, is lossy handover.Lossless packet switched handover can, according to prior art, only beachieved by operating LLC/SNDCP in acknowledged mode, which willincrease overhead, add delay and reduce overall throughput.

Consequently, there is a need of reducing data transfer delay andcontrol signaling, without risking packet losses due to packet switchedhandover.

It is consequently an object of the present invention to reduce datatransfer delay without packet losses due to handover.

A further object is to reduce packet switched data transfer overheadwithout packet losses due to handover.

It is also an object not to require LLC-AM as a means for reducing therisk of data losses at packet switched handover.

Another object is to circumvent throughput reduction due to handover.

Finally, it is an object of the present invention to enhance LLC/SNDCPprotocols to provide lossless handover with LLC/SNDCP operating inunacknowledged mode.

These objects are met by a method and system of lossless base stationhandover for packet switched communications not requiring LLC/SNDCP tooperate in acknowledged mode during the complete data transfer session.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates protocol layers for GERAN (GSM-EDGE Radio AccessNetwork) A/Gb mode, according to prior art.

FIG. 2 illustrates schematically some network elements involved inpacket switched handover.

FIG. 3 illustrates an outline of a signaling diagram associated with anexample PS handover according to a mode of the invention.

FIG. 4 illustrates a simplified block diagram of a mobile stationaccording to the invention.

FIG. 5 illustrates a simplified block diagram of a support nodeaccording to the Invention.

FIG. 6 illustrates a simplified block diagram of a base station entityaccording to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

To handle handover of packet services that require a minimum of packetloss it is currently possible to operate both the RLC and the LLC/SNDCPprotocols in acknowledged mode. However this is not desirable as withthese two protocols operating in acknowledged mode a certain amount ofdelay will be introduced whenever retransmission is determined to benecessary at any of these layers. The delay introduced especially whenretransmission occurs at the LLC layer may impact higher layeroperation, e.g. for TCP based services where a protocol stack consistingof TCP/IP/SNDCP/LLC/RLC is used, with the net result being a majorreduction in throughput temporarily being experienced by the affectedpacket service. This extra delay will impose a lowered quality ofservice as perceived by the user. In addition, operating these twoprotocols in acknowledged mode will result in increased overhead usedfor control plane functions, and therefore renders less bandwidthavailable to user plane payload.

Today's streaming services are normally implemented operating RLC inacknowledged mode and LLC/SNDCP in unacknowledged mode, which allows foreliminating the potential for serious delay problems as described above.However, this approach has the problem of not being able to supportlossless packet service for the case where MS mobility involves a changeof radio cell/base station and SGSN.

Packet switched base station handover according to the invention isassociated with radio cell change both within GERAN and between GERANand UTRAN.

To minimize potential delay and extra overhead as would result ifoperating the LLC/SNDCP protocols in acknowledged mode, for all packetflows subject to lossless packetswitched handover, PS handover, and tobe compliant with the principles used for lossless data transfer inUTRAN, the following embodiments are identified:

-   -   new mode of operation for SNDCP    -   management of downlink Status with/without Source BSS        assistance, and    -   management of uplink status with/without source BSS assistance.

They are preferably combined. The packet loss during handover isminimized without requiring LLC/SNDCP protocols to operate inacknowledged mode during an entire data transfer session. Thereby higheruser data rates are achieved during the entire data transfer session.

New Mode of Operation for SNDCP

The SNDCP protocol is modified to support a new mode of operation whereit operates with both N-PDU Send and Receive Sequence numbers combinedwith the LLC protocol operating in unacknowledged mode. This means thatthe SNDCP protocol entities in the mobile station and in the networkshall each maintain a Send and a Receive N-PDU Sequence number and alsoGTP T-PDU uplink and downlink sequence numbers for each packet flowsubject to lossless PS handover. This sequence number information isforwarded from the source SGSN to the target SGSN so that a SNDCP enginestarted in the target SGSN can maintain sequence number continuity withthe SNDCP engine used in the source SGSN. The downlink N-PDU sequencenumber and the downlink GTP T-PDU numbers are provided along with eachN-PDU forwarded from the source SGSN to the target SGSN.

Management of Downlink Status with Source BSS Assistance

The downlink LLC data buffered in the source BSS that has not yet beensent to or acknowledged by the mobile station (at the RLC layer) at thepoint of time when the source BSS sends the PS handover command messageto the MS can be deleted and a status message sent back to the sourceSGSN telling it how many LLC PDUs were deleted for each packet flowsubject to lossless PS handover.

Alternatively, the status message sent from the source BSS to the sourceSGSN could provide parts of the deleted LLC PDUs, e.g. the LLC header,or even the complete LLC PDUs. This means that SN-UNITDATA PDUspreviously sent down to LLC at the source SGSN and relayed to the sourceBSS as segmented LLC PDUs will either be explicitly returned to thesource SGSN or referenced in such a way to allow the source SGSN todetermine which N-PDUs have not been sent to the MS, i.e. which wholeN-PDUs have been acknowledged by the MS on RLC layer.

The Send N-PDU sequence numbers determined by the source SGSN are thenforwarded in a message to the target SGSN. Upon MS arrival in the targetcell the target SGSN can start transmitting the next downlink N-PDUexpected by the MS for each packet flow subject to lossless PS handover.The MS can detect duplications of downlink N-PDUs since the sequencenumber used at the target SGSN is based on the sequence number used bysource SGSN and it is included in the header of each N-PDU sent from thetarget SGSN.

For this embodiment it is required that the source SGSN supports N-PDUbuffering if downlink N-PDUs are to be forwarded to the target SGSN intheir correct order or the source BSS sends a status message that doesnot contain complete LLC PDUs. If the target SGSN can accept N-PDUs thatare out of order and the status message sent by the source BSS containscomplete LLC PDUs then the source SGSN need not support N-PDU buffering.

Management of Downlink Status without Source BSS Assistance

In this embodiment the source SGSN can only estimate the Send N-PDUsequence numbers based on the delay attribute (with sufficient marginadded) associated with each packet flow subject to lossless PS handoverand forward these estimates to the target SGSN.

Source BSS buffers has downlink LLC data not yet sent to or acknowledgedby the mobile station (at the RLC layer) at the point of time when thesource BSS sends the PS handover command message to the MS. The sourceBSS sends the PS handover command message to the MS. Thereafter it sendsa status message back to the source SGSN only indicating that the MS hasbeen sent a PS handover command message (i.e. no downlink status isincluded in the message).

If upon arrival in the target cell, the MS does not send the network amessage that provides downlink sequence number status for all packetflows subject to lossless PS handover, the target SGSN has no choice butto use the estimated Send N-PDU sequence numbers provided by the sourceSGSN. This may result in the target SGSN sending multiple downlinkN-PDUs already received by the MS. However, the MS can once again detectduplications of downlink N-PDUs since the sequence number is included inthe header of each SN-UNITDATA PDU used to transit each N-PDU.

If upon arrival in the target cell, the MS sends the network a messagethat provides downlink sequence number status for all packet flowssubject to lossless PS handover, the target SGSN will have exactknowledge as to which downlink N-PDU to begin sending for each packetflow. The target SGSN deletes all downlink N-PDUs forwarded from thesource SGSN that are implicitly acknowledged by the downlink sequencenumber status provided by the MS upon arrival in the target cell.

The embodiment requires that the source SGSN supports N-PDU bufferingsince a minimum set of downlink N-PDUs sent down to the source BSS mustbe buffered for each packet flow subject to lossless PS handover. Thequantity of N-PDUs buffered for a given packet flow can be determinedby, e.g., the delay attribute associated with that packet flow.

Management of Uplink Status with Source BSS Assistance

When the source BSS receives the PS handover command message from thesource SGSN it will at a point of time stop acknowledging RLC packets inthe uplink. When this occurs it will send a status message to the sourceSGSN indicating that no more uplink LLC PDUs will be forwarded to it.The source SGSN can then determine the Receive N-PDU sequence numbersfor all uplink packet flows subject to lossless PS handover and includethem in a message to the target SGSN. After notifying the source SGSN,the source BSS will send the PS handover command message to the MS.

The PS handover command message may contain an up to date RLC ACK/NACKreport allowing the MS to determine which N-PDUs have been completelyreceived by the network. The MS will start uplink transmission uponarrival in the target cell from the next uplink N-PDU that was notacknowledged by lower layers in the old cell. This N-PDU should alwayscorrespond to the next uplink N-PDU expected by the target SGSN for eachpacket flow subject to lossless PS handover.

Alternatively, the PS handover command message may not include an RLCACK/NACK report or any other indication of uplink status that the MScould use to determine the Send N-PDU sequence number for the packetflows subject to lossless PS handover. The MS will therefore startuplink transmission upon arrival in the target cell from what isestimated next uplink N-PDU that was not acknowledged by lower layers inthe old (source) cell. In this case the first N-PDU sent by the MS inthe new cell may not correspond to the next uplink N-PDU expected by thetarget SGSN. However, since the N-PDU sequence number is included in theheader of each SN-UNITDATA PDU used to transmit each N-PDU the targetSGSN will be able to remove any duplication.

For both alternatives the source BSS is considered to have providedassistance to the source SGSN in that a status message is sent. Themessage indicates that the source BSS has stopped acknowledging RLCpackets in the uplink and that no more uplink LLC PDUs will be forwardedto it.

Management of Uplink Status without Source BSS Assistance

The PS handover command message may be sent from the source SGSN to thesource BSS and include the expected Receive N-PDU sequence Number thatthe MS should start transmission with in the target cell for each uplinkpacket flow subject to lossless handover. This sequence numberinformation is provided by the source SGSN without conferring with thesource BSS as to whether or not it has stopped acknowledging RLC data inthe uplink. As such, additional uplink LLC PDUs may be acknowledged bythe source BSS prior to the PS handover command message being sent tothe MS and may therefore result in a conflict with the Send N-PDUsequence number as viewed by the MS. In this case the MS must alwaysaccept the uplink sequence number information provided in the PShandover command message over the uplink sequence number informationderived from local RLC operation. This means that the MS must alwaysbuffer some uplink N-PDUs that have already been confirmed according toRLC. The quantity of these N-PDUs that need to be buffered is expectedto be small.

According to the invention a sequence tracking mode, STM, is defined forSNDCP. In STM, the SNDCP entities in the SGSN and in the MS shall alwayskeep track of the uplink and downlink N-PDU sequence numberscorresponding to SNDCP in LLC-AM. Further, in STM uplink and downlinkG-PDU sequence numbers associated with uplink and downlink N-PDUs arerecorded corresponding to SNDCP in LLC-AM. Also, in STM SNDCP entitiesin SGSN and in MS shall make use of SN-UNITDATA PDUs corresponding toSNDCP in LLC-UM and SNDCP shall maintain sequence number continuity whenPS handover occurs across SGSN.

An additional case is where the PS handover command sent to the MS doesnot include any Receive N-PDU numbers for any of the uplink packet flowssubject to lossless PS handover. In this case the MS will use its localknowledge of uplink status which may lead to a duplicated uplink N-PDUbeing sent by the MS in new (target) cell. Since this duplication willbe deleted by the target SGSN, since N-PDU sequence number continuity issupported across SGSNs, it will not be a problem.

FIG. 3 illustrates an outline of a signaling diagram associated with anexample PS handover according to a mode of the invention. In the exampleLLC operates in unacknowledged mode LLC-UM, even if the invention isapplicable also in acknowledged mode, LLC-AM.

The signaling of FIG. 3 is initiated by an MS having one or more ongoingpacket flows subject to lossless PS handover when the source BSSdetermines that a PS handover is required <<1>>. RLC is operating inacknowledged mode, LLC is operating in unacknowledged mode and SNDCP isoperating in sequence tracking mode, STM. Thereby N-PDU sequencenumbering is managed as if LLC were operating in acknowledged mode.Therefore SNDCP entities in MS and network has to manage two sequenceparameters for each packet flow subject to lossless PS handover, theSend N-PDU and the Receive N-PDU sequence numbers.

In STM SNDCP shall use SN-UNITDATA PDUs as with LLC-UM. For each packetflow subject to lossless PS handover the source SGSN buffers a set ofdownlink N-PDUs that reflects the delay attribute associated with thatpacket flow. As a non-exclusive example the N-PDUs received from theGGSN during the running latest 500 ms are buffered.

The source BSS sends a PS Handover Required message <<2>> to the sourceSGSN.

The source SGSN sends a Prepare PS Handover Request message to thetarget SGSN <<3>>.

The target SGSN sends a PS Handover Request message <<4>> to the targetBSS. The target BSS pre-allocates radio resources, if available, to therequested flows and sends a PS Handover Request Acknowledge message<<4′>> back to the target SGSN.

The target SGSN sends a Prepare PS Handover Response message to thesource SGSN <<5>>. This message indicates that the SGSN is now ready toreceive downlink data forwarded from the source SGSN. When source SGSNreceives the Prepare PS Handover Response message <<5>> it

-   -   stops sending downlink data to the source BSS,    -   sends the PS Handover Command message <<6>> to the source BSS        containing among other things the N-PDU Receive Sequence number        of the next expected uplink N-PDU to be received for each packet        flow subject to lossless PS handover,    -   starts forwarding to the target SGSN all buffered downlink        N-PDUs received from the GGSN prior to the arrival of the        Prepare PS Handover Response message from the target SGSN, and    -   starts forwarding to the target SGSN downlink N-PDUs received        from the GGSN after the arrival of the Prepare PS Handover        Response message from the target SGSN <<9>>.

Each downlink N-PDU forwarded to the target SGSN <<9>> contains anassociated Send N-PDU sequence number and a GTP sequence number. Thetarget SGSN starts buffering of the forwarded downlink N-PDUs until theMS indicates its presence <<7>> in the target cell by sending a PSHandover Complete message to the target SGSN <<7>>, <<10>>.

When the source BSS receives the PS Handover Command <<6>> it

-   -   immediately stops reception and acknowledgement of data in the        uplink;    -   stops transmission of downlink data towards the MS but may        terminate transmission at an LLC PDU boundary without waiting        for an acknowledgement;    -   sends a Forward BSS Context message to the source SGSN, the        message not including information on which buffered downlink        LLC-PDUs has been discarded;    -   sends a PS Handover Command <<7>> to the MS ordering the MS to a        new target cell; the message including Receive N-PDU (uplink)        sequence number of the next expected N-PDU to be received as        viewed by the source SGSN for each packet flow subject to        lossless PS handover;

When the MS has reconfigured itself and acquired synchronization in thenew cell, it sends a PS Handover Complete message to the target BSS<<7′>>. This message includes the sequence number of the next expecteddownlink N-PDU to be received (as viewed by MS) for each downlink packetflow subject to lossless PS handover.

The source BSS then sends a Forward BSS Context message <<8>> to thesource SGSN indicating that the BSS has ordered the MS to the new cell.The message comprises no send buffer status information that the sourceSGSN could make use of for determining the precise status of transmitteddownlink N-PDUs before starting forwarding data to the target SGSN.

Upon receiving the Forward BSS Context message <<8>>, the source SGSNdetermines the following values for each packet flow subject to losslessPS handover and forwards this information to the target SGSN in theForward SRNS Context message <<9>>:

-   -   Downlink N-PDU Send sequence number for the next downlink N-PDU        to be sent to the MS,    -   Downlink GTP-U sequence number for the next downlink GTP-U T-PDU        to be relayed to the target SGSN,    -   Uplink N-PDU Receive sequence number for the next uplink N-PDU        to be received from the MS, and    -   Uplink GTP-U sequence number for the next uplink GTP-U T-PDU to        be sent from the target SGSN to the GGSN.

Prior to receiving the Prepare PS Handover Response message <<5>> thesource SGSN was buffering downlink N-PDUs according to the delayattributes of the packet flows subject to lossless PS handover. Sincethe Forward BSS Context message <<8>> received from the source BSS doesnot include downlink status information, the source SGSN selects valuesfor the two downlink sequence numbers listed above that reflect theoldest buffered downlink N-PDU for each of the packet flows subject tolossless PS handover. I.e. a worst case scenario is anticipated andcorresponding values selected.

Upon receiving the Forward SRNS Context message, the target SGSN sends aForward SRNS Context Acknowledge message <<9′>> back to the source SGSN.

Thereafter the target BSS sends a PS Handover Complete message <<10>> totarget SGSN. The PS Handover Complete message <<10>> includes thesequence number of the next expected downlink N-PDU to be received (asviewed by MS) for each packet flow subject to lossless PS handover.

The target SGSN can now start sending the buffered downlink datastarting with the next downlink N-PDU expected by the MS for each packetflow subject to lossless PS handover.

The downlink sequence number status information provided in the PSHandover Complete message <<12>> allows the target SGSN to:

-   -   delete all downlink N-PDUs forwarded from the source SGSN that        are implicitly acknowledged by the downlink sequence number        status information, and    -   ignore downlink sequence number status information provided by        the source SGSN in the Forward SRNS Context message <<9>>.

The target SGSN sends a PS Handover Complete message <<12>> to thesource SGSN, which acknowledges the completion of the handover procedureby responding with a PS Handover Complete Acknowledge message <<12′>>back to target SGSN. Target SGSN sends an Update PDP Context Request tothe GGSN <<13>>. The GGSN updates its PDP context fields and return anUpdate PDP Context Response message <<13′>>. SGSN initiates Packet FlowProcedures to release resources in the source BSS <<14>>. Finally, MSand target SGSN perform routing area update procedure <<15>>.

The example signaling described above in relation to figure illustratesa method and system where source SGSN provides the MS with the sequencenumber of the next expected uplink N-PDU to be received in the PSHandover Command <<6>>, <<7>>. (Management of uplink status is providedwithout information processing of source BSS.)

MS preferably provides the network with the sequence number of the nextexpected downlink N-PDU to be received in the PS Handover Completemessage <<7′>>, <<10>>. (Management of downlink status is providedwithout information processing of source BSS).

SNDCP entities in the source SGSN preferably support some buffering ofdownlink N-PDUs. The source SGSN then buffers an amount of N-PDUscorresponding to the delay attribute of the associated packet flow. Uponcompletion of the PS handover preparation phase all such buffered N-PDUscan be forwarded to the target SGSN to ensure all forwarded N-PDUsarrive in correct order. Downlink N-PDUs received from the GGSN after PShandover preparation is completed will be sent to the target SGSN afterthe buffered downlink N-PDUs are forwarded. Upon MS arrival to the newcell, the target SGSN discovers the downlink status of the packet flows,e.g. via information provided in the PS Handover Complete message<<10>>, begins transmitting the appropriate downlink N-PDU for eachpacket flow subject to lossless PS handover and deletes all implicitlyconfirmed downlink N-PDUs received from the source SGSN.

In SNDCP STM, SNDCP entities in the target SGSN will be required tosupport some buffering of downlink N-PDUs. This is necessary as losslessoperation requires that the target SGSN be informed of the presence ofthe MS in the new cell before it can begin downlink transmission ofpacket data.

The source BSS also supports buffering which allows it the option ofattempting to empty downlink buffers upon receiving a PS HandoverCommand <<6>> from the source SGSN.

In SNDCP STM, SNDCP entities in MS will be required to buffer the uplinkN-PDUs beyond the point where their associated RLC/MAC entitiesacknowledges the complete transmission of any given LLC PDU. This isnecessary for the example signaling where management of uplink status isprovided without information processing of source BSS. This bufferingallows source BSS the option of continued reception of uplink data whenattempting to empty downlink buffers upon reception of a PS HandoverCommand <<6≦> from the source SGSN.

The example described in relation to FIG. 3 is just an example. Itillustrates, e.g., source SGSN and target SGSN as separate entities(inter SGSN PS handover). However, the invention also covers intra SGSNPS handover between base stations. Also, in FIG. 3 signaling for onesingle radio access technology, RAT, is illustrated. Though, the sameprinciples are valid also for inter RAT PS handover, such as for PShandover between base stations of GERAN and UTRAN respectively.

FIG. 4 illustrates a simplified block diagram of a mobile stationaccording to the invention. The mobile station comprises processingmeans <<μ_(MS)>> operating according to one or more protocols forcommunicating protocol data units as described above. Receive means<<R_(MS)>> receives information from the communications network to whichthe mobile station is connected. The receive means are connected to theprocessing means <<μ_(MS)>> and receives, e.g., information from thecommunications network on next expected uplink protocol data unit athandover. Buffer means <<B_(MS)>> buffers uplink protocol data units,N-PDUs, as described above.

FIG. 5 displays schematically a block diagram of a support node, such asa Serving GPRS Support Node, SGSN, according to the invention. Thesupport node comprises processing means <<μ_(SN)>> operating accordingto one or more protocols for communicating packet switched data asdescribed above. Receive means <<R_(SN)>> receives protocol data unitsfrom one or more respective mobile stations on next expected downlinkprotocol data unit, N-PDU. Transmit means <<T_(SN)>> transmits protocoldata units to the one or more mobile stations which are communicatingpacket switched data over the SGSN. Buffer means <<B_(SN)>> buffersdownlink N-PDUs.

FIG. 6 depicts a schematic block diagram of a base station entityaccording to the invention. The base station entity comprises receivemeans <<R_(BS)>>, transmit means <<T_(BS)>> and buffer means <<B_(BS)>>.The receive means <<R_(BS)>> receiving one or more commands of basestation change as decided by network to which the base station entity isconnected. Also, receive means receives <<R_(d,BS)>> that is notnecessarily identical to receive means <<R_(BS)>>, receives uplink datafrom one or more mobile stations communicating packet switched data viathe base station entity. The transmit means <<T_(BS)>> transmitsprotocol data units to one or more mobile stations communicating packetswitched data via the base station entity.

A person skilled in the art readily understands that the properties ofan SGSN, a GGSN, a BSS, a base station or an MS are general in nature.The use of concepts such as SGSN or MS within this patent application isnot intended to limit the invention only to devices associated withthese acronyms. It concerns all devices operating correspondingly, orbeing obvious to adapt thereto by a person skilled in the art, inrelation to the invention. As an explicit non-exclusive example theinvention relates to mobile equipment without a subscriber identitymodule, SIM, as well as mobile stations including one or more SIMs.Further, protocols and layers are referred to in close relation withGPRS, UMTS and Internet terminology. However, this does not excludeapplicability of the invention in other systems with other protocols andlayers of similar functionality.

The invention is not intended to be limited only to the embodimentsdescribed in detail above. Changes and modifications may be made withoutdeparting from the invention. It covers all modifications within thescope of the following claims.

We claim:
 1. A method for base station handover of a mobile station (MS)from a source cell of a source base station to a target cell of a targetbase station in a cellular radio network, wherein the base stationstransfer packet switched communications between the mobile station andthe network, the method comprising: by the source base station while theMS is in the source cell: receiving a handover command message from asource support node, stopping reception and acknowledgement of uplinkpackets from the MS; stopping transmission of downlink packets towardsthe MS; sending a context message to the source support node withoutindicating which buffered downlink packets have been discarded;transmitting to the MS, the handover command message ordering the MS toconnect to the target cell; and transmitting to the MS, informationregarding a sequence number of a next expected uplink packet that isexpected by the source support node for a packet flow subject tolossless packet switched handover; and by the MS upon arrival andsynchronization of the MS in the target cell: transmitting to the targetbase station, information regarding a sequence number of a next expecteddownlink packet that is expected by the MS for the packet flow subjectto lossless packet switched handover; and starting uplink datatransmission to the target base station beginning with the next expecteduplink packet that is expected by the source support node.
 2. The methodaccording to claim 1, wherein the MS also receives a transfer of radiolink control data blocks in an acknowledged mode.
 3. The methodaccording to claim 1, wherein the information transmitted by the MS tothe target base station includes a sequence number of the next expectedProtocol Data Unit (PDU) to be received by the MS.
 4. The methodaccording to claim 1, wherein the MS receives lower layer packetacknowledgements, and the step of starting uplink data transmissionincludes starting uplink data transmission in the target cell beginningwith the next up-link packet that was not acknowledged by lower layersin the source cell.
 5. A system for base station handover of a mobilestation (MS) from a source cell of a source base station to a targetcell of a target base station in a cellular telecommunication network,wherein the base stations transfer packet switched communicationsbetween the MS and the network, the system comprising: a first processorin the source base station configured to cause the source base stationto perform the following functions while the MS is in the source cell:receive a handover command message from a source support node; stopreception and acknowledgement of uplink packets from the MS; stoptransmission of downlink packets towards the MS; send a context messageto the source support node without indicating which buffered downlinkpackets have been discarded; transmit to the MS, the handover commandmessage ordering the MS to connect to the target cell; and transmit tothe MS, information regarding a sequence number of a next expecteduplink packet that is expected by the source support node for a packetflow subject to lossless packet switched handover; and a secondprocessor in the MS configured to cause the MS to perform the followingfunctions upon arrival and synchronization of the MS in the target cell:transmit to the target base station, information regarding a sequencenumber of a next expected downlink packet that is expected by the MS forthe packet flow subject to lossless packet switched handover; and startuplink data transmission to the target base station beginning with thenext expected uplink packet that is expected by the source support node.6. The system according to claim 5, wherein the MS is configured toreceive a transfer of radio link control data blocks in an acknowledgedmode.
 7. The system according to claim 5, wherein the informationtransmitted by the MS to the target base station includes a sequencenumber of the next expected Protocol Data Unit (PDU) to be received bythe MS.
 8. The system according to claim 5, wherein the MS is configuredto receive lower layer packet acknowledgements, and the step of startinguplink data transmission includes starting uplink data transmission inthe target cell beginning with the next up-link packet that was notacknowledged by lower layers in the source cell.